Abstract | Impact structures on Mercury are predominant surface land-forms. They represent important records regarding the magnitude and timing of the “Late Heavy Bombardment” in the early inner solar system.
Geophysical data in combination with the surface morphology of impact basins may help improve our understanding of formation processes of basins and alterations with time. In particular, gravity anomalies may hint at complex mass and density distributions in the upper crust of the planet.
In this study we are using gravity and topography data obtained by MESSENGER spacecraft to create a catalogue of impact basins on Mercury larger than 150 km in diameter. 311 basins were tentatively identified and mapped. Complementary MESSENGER data sets were used to identify the basins based on morphology (USGS shaded relief maps, stereo image DTMs) and gravity signal. The confidence of identification (certain or probable) was assessed. We determined the location, diameter and depth of each basin. Moreover, we applied a classification according to type (from simple to complex) degradation state, as well as visibility of central peaks, rims and terraces.
An in-depth analysis of gravity and topography data sets was carried out to understand the information held in spherical harmonic models of different degree and order. Previously mapped impact basins from our inventory and their surroundings were characterized with respect to their local gravity field signals. Impact basins were found to show more complex gravity- and topography signatures with increasing diameters. Correlation analysis was carried out between topography and gravity signatures of individual impact structures. While, smaller basins possess a negative gravity anomaly, this values increases to the positive range with increasing basin diameter. The positive gravity signals may display high mass and density concentrations, which may be caused by an uplift of mantle material after the crater excavation phase. The excavation was followed by an isostatic adjustment caused by cooling and contraction of the melt pool. According to this, basins associated with a positive gravity anomaly may possess a thinner crust.
We selected a subset of impact basins from our inventory representing the variety of morphologies and gravity signatures. Due to MESSENGERS’s highly eccentric orbit, resulting in gravity data of limited resolution in the southern hemisphere, this study is focusing on impact structures in the northern hemisphere only. Measurements by the upcoming Bepi-Colombo will provide a significant improvement in the gravity and topography models. The Bepi-Colombo Laser Altimeter (BELA) will produce accurate topographic profiles which will allow to characterize Mercury’s impact basin in great detail. We assessed the expected coverage of basins by BELA profiles after two years of operation at Mercury. |
